Copter: build return path in run() rather than init()

This avoids attempting to build a return path if we don't currently have
a home or origin

ArduCopter/mode.h

@@ -1027,7 +1027,9 @@ private:
     } rtl_path;
 
     // Loiter timer - Records how long we have been in loiter
-    uint32_t _loiter_start_time = 0;
+    uint32_t _loiter_start_time;
+
+    bool path_built;
 };
 
 

ArduCopter/mode_rtl.cpp

@@ -19,7 +19,7 @@ bool Copter::ModeRTL::init(bool ignore_checks)
     }
     // initialise waypoint and spline controller
     wp_nav->wp_and_spline_init();
-    build_path(!copter.failsafe.terrain);
+    path_built = false;
     climb_start();
     return true;
 }
@@ -39,6 +39,11 @@ void Copter::ModeRTL::restart_without_terrain()
 // should be called at 100hz or more
 void Copter::ModeRTL::run(bool disarm_on_land)
 {
+    if (!path_built) {
+        path_built = true;
+        build_path(!copter.failsafe.terrain);
+    }
+
     // check if we need to move to next state
     if (_state_complete) {
         switch (_state) {